Referring to FIG. 1, the present invention is an improved airborne radar detection system 100 which is intended to accurately determine the direction of a radar emitting object 110. The radar emitting object 110 is sometimes called an "emitter". Such systems typically have four transducers 120, 122, 124 and 126 mounted at reference angles of 45 degrees, 135 degrees, 225 degrees and 315 degrees, respectively, relative to the forward direction of a plane 130. Such system configurations can be used in a variety of radar warning systems, typically in systems which warn the pilot of the direction (i.e., radar signal angle of arrival) and distance of nearby radars.
The goal of the radar detection system 100 is to accurately determine the "angle of arrival" of a radar emitting object 110. The angle of arrival is typically defined to be the azimuth angle of a direct line to the radar 110. Such systems are also used to determine the approximate distance of the radar 110 from the plane 130. Prior art systems generally compute the angle of arrival by first determining which two transducers 120-126 are receiving the strongest signals from the radar 110, and then using the relative strengths of these two signals to determine the azimuth angle to the radar.
This prior art method of computing an azimuth angle is accurate when the radar emitting object is in the same plane as the four transducers 120-126, and is reasonably accurate when the angle of elevation to the radar, with respect to the plane occupied by the transducers, is fairly small. However, the inventors have found that the azimuth angles generated by prior art systems are quite inaccurate when the angle of elevation is more than thirty degrees or so.
One reason that such inaccuracies have been accepted in prior art systems is that it has been assumed that correcting the inaccuracy would require additional transducers, extra circuitry, exceed the allocated volume, and would slow down the response time of the system. In many such radar detection systems, fast determination of the azimuth angle of arrival is very important and speed cannot be sacrificed for better accuracy.
The present invention is based on the discovery by the inventors that using the measurements from three transducers, one can very accurately determine the azimuth angle to an object as well as the angle of elevation. In addition, the inventors have found a way of performing the necessary computations using very high speed hard-wired logic, thereby providing more accurate angle of arrival measurements without any sacrifice in the speed at which the measurements are performed.
It is therefore the primary object of the present invention to provide a radar detection system which accurately determines the angle of arrival of a radar, even when that angle includes a substantial elevation angle. Another object of the present invention is to provide a system which can determine the angle of arrival of a radar signal accurately and quickly. Yet another object of the present invention is to provide a system which can accurately determine the direction of an object using a standard set of four radar transducers.